This invention relates to lip seals and methods of forming and installing them. A preferred application of the lips seals of the invention will be as clutch seals in the automatic transmission of an automobile.
Lip seals are found in applications where an internal seal is needed against a unidirectional fluid flow. This could be, for example, where fluid pressure forces a piston in one direction and the piston is returned by the force of a spring. Generally, the fluid is confined within a chamber and it is the function of the lip seal to prevent the fluid from leaking past the piston via the annular space between the piston and the interior chamber wall.
Automotive clutch seals are reciprocating seals where pressure is applied from one direction and the piston is returned by means of springs. This action results in the activation of various clutch packs.
In addition to automotive clutch seals, lip seals can be used in a multitude of applications. Further, each application may involve a number of lip seals.
Consider, for example, the large number of automobiles produced every year. Each of these may include an automatic transmission wherein clutch seals are employed. An improvement in lip seals generally, or even clutch seals specifically, which resulted in a small savings in the cost of each seal can become significant when multiplied by the large number of seals employed. An improvement which resulted in an even greater savings per seal would therefore be of even greater economic significance.
Molded rubber lip seals have generally been employed in the past as the seals for unidirectional fluid flow seals. Rubber seals, though, suffer from defects such as thermal embrittlement and stick-slip conditions, are not possessed of an ability to wear for long periods, and have a high coefficient of friction.
Molded or machined polytetrafluoroethylene (hereinafter referred to as PTFE) lip seals overcome some of the defects of the molded rubber lip seals in that they have superior resistance to thermal embrittlement, resistance to stick-slip conditions, and produce less friction resistance and longer wear. However, PTFE is an expensive material of construction for making lip seals, and when such seals are made by the conventional molding process, they are not economically competitive with molded rubber seals.
Because of the expense of the PTFE lip seal, a need exists for a PTFE lip seal which is lower in cost but which retains the superior qualities of PTFE.